In a gas chromatograph mass spectrometer (GC-MS) or liquid chromatograph mass spectrometer (LC-MS) in which a mass spectrometer is used as the detector for a gas chromatograph (GC) or liquid chromatograph (LC), a measurement on a sample temporally separated into components by a column in the chromatograph is repeatedly performed in the mass spectrometer. For example, when a scan measurement over a predetermined range of mass-to-charge ratios m/z is performed in the mass spectrometer, a set of data showing a mass spectrum within the predetermined range of mass-to-charge ratios is obtained in each scan measurement. In a mass spectrum which is obtained when a sample containing a specific component has been introduced into the mass spectrometer, one or more peaks originating from that component appear. Accordingly, it is possible to identify an unknown component by analyzing the pattern of the peaks appearing on the mass spectrum, in addition to the elution time (i.e. retention time) of that component in the chromatograph.
There are various types of mass spectrometers, such as the quadrupole, time of flight, ion trap, or Fourier transform ion cyclotron resonance type. In any of those types, a mass discrepancy (the difference between measured and true mass-to-charge ratios) occurs due to various factors, such as a change in the ambient temperature. Normally, such a mass discrepancy is calculated based on the result of a measurement on a specific substance whose exact mass-to-charge ratio is previously known (e.g. a standard substance), and a correction is performed in the data processing so as to eliminate the calculated mass discrepancy. In a chromatograph mass spectrometer, it is often the case that the amount of mass discrepancy changes with the passage of time from the introduction of the sample into the chromatograph. When a high level of analysis accuracy is needed, such a mass discrepancy whose amount changes with the passage of time needs to be corrected in almost real time. A method for correcting such a mass discrepancy is described in Patent Literature 1, in which a mass spectrometric analysis is performed with an internal standard substance continuously added to the sample separated into components by the chromatograph, and the mass-to-charge ratio at which the internal standard substance is detected is used to correct the mass discrepancy.
Even when such a correction of the mass discrepancy is possible, a situation in which the mass discrepancy becomes extremely large in the middle of the measurement should not be considered as normal; in such a situation, it is likely that some problem has occurred. Accordingly, in some cases, it is desirable to observe the temporal change in the amount of mass discrepancy or in a mass correction quantity for correcting the mass discrepancy, so as to confirm or verify whether or not the measurement has been properly performed. For this purpose, a conventional device has been known which can display the relationship between the retention time and the mass correction quantity in a graphical form, for example, as shown in FIG. 4 (see FIG. 4 in Non Patent Literature 1, page 9 of Non Patent Literature 2, or other documents). Such a graph allows analysis operators to instantly comprehend whether or not the mass correction quantity has reached or even exceeded a specific value in the middle of the measurement. This is convenient for checking the reliability of the measurement.
As one technique of the GC analysis, a technique called the “comprehensive two-dimensional GC” or “GC×GC” has been commonly known (see Patent Literature 2 or other documents). In the comprehensive two-dimensional GC, various components contained in a target sample are initially separated by a column which is the first dimension (which is hereinafter called the “primary column”), and the thereby eluted components are introduced into a modulator. The modulator repeats an operation including the steps of capturing the introduced components at regular intervals of modulation time (which is normally within a range from a few seconds to approximately one dozen seconds), detaching those components with an extremely narrow time bandwidth, and introducing them into a column which is the second dimension (which is hereinafter called the “secondary column”). The component separation in the primary column is normally performed under such a separation condition that the elution occurs at a rate approximately equal to or slightly lower than the rate in a commonly used GC. On the other hand, as compared to the primary column, the column used as the secondary column has a different polarity, shorter length and smaller inner diameter, with the component separation performed under such a condition that each elution will be completed within the specified modulation time. In this manner, in the comprehensive two-dimensional GC, a plurality of components which have not been separated by the primary column and whose peaks overlap each other can be separated in the secondary column, whereby the separation performance is dramatically improved as compared to normal GCs.
A similar technique to the comprehensive two-dimensional GC is also known in the field of LC analysis, which uses two columns having different separation characteristics and is called the “comprehensive two-dimensional LC” or “LC×LC”. In the present description, both the comprehensive two-dimensional GC and the comprehensive two-dimensional LC are collectively called the “comprehensive two-dimensional chromatograph”.
In those comprehensive two-dimensional chromatographs, the retention time in the secondary column (“second retention time RT2”) is an enlargement of a narrow range of time in the retention time in the primary column (“primary retention time RT1”). Therefore, the measured result may be represented by a chromatogram similar to the normal one. However, in many cases, using a one-dimensional chromatogram makes it difficult to comprehend the state of separation in each individual column, since the two columns have different separation characteristics. Therefore, in order to present the state of separation in each column in an easily comprehensible form, a two-dimensional chromatogram having two orthogonal axes which respectively represent the primary retention time RT1 and the secondary retention time RT2 is created, with the signal intensity represented by contour lines, color scale, or gray scale. A commonly known data processing software product for creating two-dimensional chromatograms is “GC Image”, offered by GC Image LLC (see Non Patent Literature 3).
In recent years, mass spectrometers have been popularly used as detectors for comprehensive two-dimensional GCs or LCs. In such a comprehensive two-dimensional chromatograph mass spectrometer, if the correction of the mass discrepancy is performed in substantially real time, it is certainly possible to draw the relationship between the retention tine and the amount of mass discrepancy or mass correction quantity as shown in FIG. 4. However, if the mass discrepancy information is displayed as shown in FIG. 4, users cannot easily comprehend the relationship between the retention time and the mass discrepancy for each of the two independent columns, since, in many cases, the two columns used in a comprehensive two-dimensional chromatograph have different separation characteristics, as noted earlier. Such a problem is not unique to the mass discrepancy information; for example, a similar problem occurs with a retention-time discrepancy (i.e. the discrepancy between measured and true retention times) in a chromatograph.
Besides, in an imaging mass spectrometer frequently used for a measurement of biological samples, as disclosed in Patent Literature 3, a mass spectrum or MSn spectrum can be obtained for each of a large number of micro areas within a two-dimensional area on a sample. Based on the measured results, a mapping image which shows the distribution of the signal intensity at a specific mass-to-charge ratio corresponding to the two-dimensional area to be analyzed can be created. For example, such a device sequentially performs a mass spectrometric analysis for each of the large number of micro areas while gradually changing the position of the sample. Therefore, a considerable amount of time is needed from the beginning to the end of the measurement for one sample. During this period of time, a change in the amount of mass discrepancy may possibly occur. However, none of the conventional imaging mass spectrometers have been provided with the function of displaying, in an easily comprehensible form, the amount of mass discrepancy or mass correction quantity in a mass spectrometric analysis at each micro area.
Furthermore, no conventional comprehensive two-dimensional chromatograph or imaging mass spectrometer has the function of displaying, in an intuitively comprehensible form for analysis operators, such kinds of information as a difference between analysis results obtained for a plurality of samples, difference between analysis results obtained for different components in one sample, difference in various physical or statistical quantities respectively calculated from the analysis results obtained for a plurality of samples, or difference in various physical or statistical quantities respectively calculated from the analysis results obtained for different components in one sample.